1. Field of the Invention
This application relates to the field of measuring a variable admittance and more particularly to the field of measuring a level in vessels with a capacitance sensor.
2. Description of Related Art
Capacitive level sensors employed in practice have a length of between several centimeters and approximately 30 meters. The dielectric constant of the products to be measured varies between approximately 1.5 and 100, and the products may also be electrically conducting. This produces a difference in the capacitance between a covered portion of the sensor and an uncovered portion of the sensor of between approximately 6 pF and 3,000 pF. The capacitance and admittance measurement apparatus which processes the sensor results are therefore preferably adjustable over a variable capacitance range (Turn-Down) of approximately 1:500.
Conventional devices use a combination of two different arrangements. According to one arrangement, the measurement result is amplified with a variable amplification and/or expanded (either using analogue amplifier stages or digitally using a microprocessor). According to another arrangement, the capacitance measurement range of the admittance measuring device is switched.
German patent DE 42 35 243 C1 by the Applicant discloses a method for switching the capacitance measurement range. The supply voltage of the sensor and thereby the sensitivity of the capacitance measurement is herein adjusted, for example, with a ratio of 1:5:25 by a three-stage switch which can be used to connect the sensor to three different winding tabs of the oscillator transmitter.
This conventional approach is disadvantageous in that the switching device is part of an extrinsic circuit operating at a different potential. Moreover, a bipolar high frequency alternating current has to be switched, wherein the turn-on resistance of the circuit element should be kept below 1 ohm to eliminate phase shifts in the measurement circuit. The self-capacitance of the switching element should also be less than a few pF to minimize adverse effects on the measured value.
As a result, a mechanical switch should be used as a switching element. This conventional approach does not enable microprocessor-controlled electronic switching of the capacitance measurement range.
It is therefore desirable to improve the conventional method for switching the measurement range of admittance level sensors so as to enable automatic electronic switching of the measurement range, while at least maintaining the switching ratio attainable with a mechanical switch. It is also desirable to provide an apparatus for automatically switching the measurement range.